In conventional plasma etching processes, charged plasma species, such as ions, and neutral radical plasma species, such as chlorine atoms, are used to bombard a substrate. This process is often mediated by a layer formed on the substrate that comprises both incident and substrate species. If the resulting reaction byproducts are volatile, such processes are capable of etching substrates in a fairly rapid manner.
However, the speed of conventional plasma etching processes comes at the expense of precision, which is manifested as feature distortion in the resulting product. Such feature distortion has a negative impact on product performance and product yield. Product yield is further adversely affected by etch speed non-uniformity across the wafer, especially at faster etch rates.
In the past, the amount of feature distortion attendant to plasma etching was somewhat negligible relative to the typical dimensions of feature sizes. However, as feature sizes have begun to approach 20-30 nm or smaller, such distortion has become increasingly significant, and indeed has emerged as a limitation to the feature sizes attainable with plasma etching. The limitations on precision attainable with conventional plasma etching processes have been further highlighted by the emerging need in the industry to process very small features on very large substrates (e.g., on wafers with diameters of 300 mm diameter or larger), which tends to magnify any loss of precision due to any non-uniformities in large area plasmas.
Atomic layer etching represents the other extreme of known etching processes. In a typical embodiment of such a process, a monolayer of chlorine is deposited on a substrate using chlorine or a chlorine-containing plasma. The monolayer of chlorine may then be bombarded with ions to etch away this layer along with a single monatomic layer of the underlying substrate. This process is repeated many times until the desired feature dimensions have been attained. Since the substrate is etched one monolayer at a time, the precision attainable with this process is extremely high.
Unfortunately, the remarkable precision afforded by atomic layer etching comes at the expense of speed. In particular, since atomic layer etching removes unwanted portions of the substrate at the rate of mere angstroms per etch, and since current features sizes are on the order of nanometers, the use of such a process to achieve typical feature sizes requires a large number of deposition/etch cycles. Consequently, the amount of time required to achieve feature definition by atomic layer etching in current technology nodes is frequently on the order of several hours, making it unsuitable for commercial implementation.
There is thus a need in the art for an etching process that exhibits better precision than conventional plasma etching processes, that is self-limiting (to accommodate any spatial non-uniformity associated with processing over very large substrates), and that offers faster etch rates than conventional atomic layer deposition. These and other needs may be met by the devices and methodologies described herein.